This invention has for object new thermosetting polyarylquinoxaline resins, which can be used for manufacturing insulation materials, particularly for enameling electric conductors adapted for use under conditions requiring increased heat stability and good resistance to thermal shock. It concerns particularly the synthesis of polymers which can be used over long periods at temperatures between 250.degree. and 350.degree. C. or over shorter periods at temperatures between 350.degree. and 450.degree. C. The polymers of the invention are also insensitive to the solvents which may contact the enameled conductor, such as ketones, alcohols, aliphatic and aromatic hydrocarbons, halogenated hydroarbons or phenol solvents.
The invention more particularly contemplates straight-chain polyarylquinoxaline resins having biphenylene and/or biquinoxalylene linkages, either inside the macromolecular chain or at the ends thereof, or again in both positions.
It is known that the polyarylquinoxalines prepared by condensation of aromatic tetra-amines with aromatic bis-.alpha.-diketones are usually linear and thermoplastic thermostable polymers, soluble in certain organic solvents. Since they hve very high molecular weights, these polymers yield highly viscous solutions, even at concentrations ranging between 5 and 10% by weight in phenolic solvents. Since the viscosity may attain several hundreds poises at a 10% concentration, such solutions cannot be used for the coating of electric conductors. When decreasing the polymer concentration to a very low level of about 4 to 6%, it is possible to enamel the electric conductors, but a sufficient thickness of the insulator cannot be attained without multiplying unreasonably the number of passages of the electric conductor in the impregnation solution. Even in these conditions, the very high molecular weight of the polyarylquinoxalines is disadvantageous, since the polymer does not adhere to the metal properly and an insulating sheath forms, which slips easily along the conductor. On the contrary, the enameling varnish must adhere to the metal and constitute with the latter a flexible and solid assembly, capable to withstand high mechanical stresses.
It is also known that the polyarylquinoxalines of lower molecular weight have a too low thermoplasticity temperature for use at 300.degree. C. or above. It has also been observed that the limitation to sufficiently low molecular weights when preparing enameling solutions having a viscosity between 3000 and 6000 cP for a 15 to 20% polymer concentration in the enameling solvent, yields varnishes having poor film properties, without flexibility and whose adherence to the metal is low.